Flat panel display device with oxide thin film transistor and method of fabricating the same

ABSTRACT

A flat panel display device with an oxide thin film transistor is disclosed which includes: an oxide semiconductor layer which has a width of a first length and is formed on a buffer film; a gate insulation film which has a width of a second length and is formed on the oxide semiconductor layer; a gate electrode which has a width of a third length and is formed on the gate insulation film; an interlayer insulation film formed on the entire surface of the substrate provided with the gate electrode; source and drain electrodes formed on the interlayer insulation film and connected to the oxide semiconductor layer; a pixel electrode formed on a passivation film and connected to the drain electrode. The first length is larger than the second length and the second length is larger than the third length.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119(a) ofKorean Patent Application No. 10-2013-0060259 filed on May 28, 2013which is hereby incorporated by references in its entirety.

BACKGROUND

1. Field of the Disclosure

The present application relates to a flat panel display device, and moreparticularly to a flat panel display device with oxide thin filmtransistor and a fabricating method which are adapted to securereliability and structural stability of elements.

2. Description of the Related Art

The flat panel display device such as a liquid crystal display devicedrives a display element using an active element, such as a thin filmtransistor, which is disposed in each pixel. Such a driving mode of thedisplay device is called as an active matrix driving mode. In the activematrix driving mode, the thin film transistor is disposed in each pixeland is used to drive the respective pixel.

Currently, a thin film transistor with a channel layer formed from oneof an amorphous silicon film and a polycrystalline silicon film is beingmost widely used.

The amorphous silicon film can be thinly deposited at a lowertemperature than 350° C. However, the mobility of amorphous silicon isslow below 0.5 cm²/Vs. Due to this, it is difficult for amorphoussilicon to realize high definition and high driving speed in asuper-sized screen. On the other hand, polycrystalline silicon has ahigh mobility of several tens through several hundreds cm²/Vs.

In view of this point, the amorphous silicon thin film transistor isutilized for driving the pixel, and the polycrystalline thin filmtransistor is utilized for driving and controlling the entire screen orpicture.

FIG. 1 is a cross-sectional view illustrating a flat panel displaydevice with polycrystalline silicon thin film transistors and afabricating method thereof according to the related art.

Referring to FIG. 1, the flat panel display device using apolycrystalline thin film transistor includes a thin film transistor 30formed on a substrate 11, and a pixel electrode 21 connected to the thinfilm transistor 30.

The thin film transistor 30 includes a buffer film 12 formed on theentire surface of the substrate 11, a semiconductor layer 13 formed onthe buffer film 12, and a gate insulation film 15 formed on the entiresurface of the buffer film 12 in such a manner as to cover thesemiconductor layer 13. Also, the thin film transistor 30 includes agate electrode 16 a formed on the gate insulation film 15, an interlayerinsulation film 17 formed on the entire surface of the gate insulationfilm 15 which is provided with the gate electrode 16 a, and source anddrain electrodes 18 a and 18 b formed on the interlayer insulation film17 and connected to source and drain regions 14 a and 14 b of thesemiconductor layer 13, respectively.

The flat panel display device further includes a passivation film 19formed on the interlayer insulation film 17 provided with the source anddrain electrodes 18 a and 18 b. The pixel electrode 21 is formed on thepassivation film 19 and connected to the drain electrode 18 b through acontact hole 20 which is formed in the passivation film 19.

Also, the method of fabricating such a flat panel display deviceincludes sequentially forming the buffer film 12, the semiconductorlayer 13, the gate insulation film 15 and the gate electrode 16 a on thesubstrate 11, injecting a dopant into the semiconductor layer 13, andsequentially forming the interlayer insulation film 17, the source anddrain electrodes 18 a and 18 b, the passivation film 19 and the pixelelectrode 21 on the gate insulation film 15 which is provided with thegate electrode 16 a.

The semiconductor layer 13 formed from polycrystalline silicon can beobtained by crystallizing the amorphous silicon layer. Thepolycrystalline silicon semiconductor layer 13 obtained from thecrystallization of amorphous silicon can have a comparative satisfactorycrystallinity, but must be treated at a high temperature of above 1000°C.

The doping procedure for the semiconductor layer 13 includes forming lowdensity regions using the gate electrode 16 a as a mask, and forminghigh density regions using a photoresist pattern, which exposes portionsof the semiconductor layer 13 corresponding to the source and drainregions 14 a and 14 b, as another mask. The low density regions (notshown) are used to reduce an off-current of the thin film transistor 30.The high density regions become the source and drain regions 14 a and 14b.

However, the formation of high density regions using the photoresistpattern, which exposes the portions of the semiconductor layer 13corresponding to the source and drain regions 14 a and 14 b, as a maskrequires an additional mask procedure with the exception of a maskprocedure for forming the gate electrode 16 a.

BRIEF SUMMARY

Accordingly, embodiments of the present application are directed to aflat panel display device and a fabricating method thereof thatsubstantially obviate one or more of problems due to the limitations anddisadvantages of the related art.

The embodiments are to provide a flat panel display device with an oxidethin film transistor adapted to secure an effective channel length, anda method of fabricating the same.

Also, the embodiments are to provide a fabricating method of a flatpanel display device with an oxide thin film transistor which is adaptedto reduce the number of mask procedures.

Additional features and advantages of the embodiments will be set forthin the description which follows, and in part will be apparent from thedescription, or may be learned by practice of the embodiments. Theadvantages of the embodiments will be realized and attained by thestructure particularly pointed out in the written description and claimshereof as well as the appended drawings.

According to a general aspect of the present embodiment for solvingproblems of the related art, a flat panel display device includes: abuffer film formed on a substrate; an oxide semiconductor layer whichhas a width of a first length and is formed on the buffer film; a gateinsulation film which has a width of a second length and is formed onthe oxide semiconductor layer; a gate electrode which has a width of athird length and is formed on the gate insulation film; an interlayerinsulation film formed on the gate electrode over the entire surface ofthe substrate; source and drain electrodes formed on the interlayerinsulation film and connected to the oxide semiconductor layer; apassivation film formed on the source and drain electrodes over theentire surface of the substrate; and a pixel electrode formed on thepassivation film and connected to the drain electrode. The first lengthis larger than the second length and the second length is larger thanthe third length.

A fabricating method of a flat panel display device with an oxide thinfilm transistor according to another general aspect of the presentembodiment for solving problems of the related art includes: forming abuffer film on a substrate; forming an oxide semiconductor layer with awidth of a first length on the buffer film; forming a gate insulationfilm with a width of a second length and a gate electrode with a widthof a third length on the oxide semiconductor layer; forming source anddrain regions by metallizing the oxide semiconductor layer using thegate insulation film; forming an interlayer insulation film on the gateelectrode over the entire surface of the substrate; forming source anddrain electrodes, which are respectively connected to the source anddrain regions, on the interlayer insulation film; forming a passivationfilm on the source and drain electrodes over the entire surface of thesubstrate; and forming a pixel electrode, which is connected to thedrain electrode, on the passivation film, wherein the first length islarger than the second length and the second length is larger than thethird length.

Other systems, methods, features and advantages will be, or will become,apparent to one with skill in the art upon examination of the followingfigures and detailed description. It is intended that all suchadditional systems, methods, features and advantages be included withinthis description, be within the scope of the present disclosure, and beprotected by the following claims. Nothing in this section should betaken as a limitation on those claims. Further aspects and advantagesare discussed below in conjunction with the embodiments. It is to beunderstood that both the foregoing general description and the followingdetailed description of the present disclosure are exemplary andexplanatory and are intended to provide further explanation of thedisclosure as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the embodiments and are incorporated herein andconstitute a part of this application, illustrate embodiment(s) of thepresent disclosure and together with the description serve to explainthe disclosure. In the drawings:

FIG. 1 is a cross-sectional view illustrating a flat panel displaydevice with polycrystalline silicon thin film transistors and afabricating method thereof according to the related art

FIG. 2 is a planar view showing a flat panel display device with anoxide thin film transistor according to an embodiment of the presentdisclosure;

FIG. 3 is a cross-sectional view showing the flat panel display devicetaken along a line I-I′ in FIG. 2;

FIGS. 4A through 4E are cross-sectional views illustrating step by stepa fabricating method of a flat panel display device with an oxide thinfilm transistor according to an embodiment of the present disclosure;and

FIG. 5 is a data sheet illustrating enhanced properties of an element byapplying the fabricating method of the flat panel display device withthe oxide thin film transistor in accordance with an embodiment of thepresent disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentdisclosure, a flat panel display device with an oxide thin filmtransistor and a fabricating method thereof (examples) of which areillustrated in the accompanying drawings. These embodiments introducedhereinafter are provided as examples in order to convey their spirits tothe ordinary skilled person in the art. Therefore, these embodimentsmight be embodied in a different shape, so are not limited to theseembodiments described here. In the drawings, the size, thickness and soon of a device can be exaggerated for convenience of explanation.Wherever possible, the same reference numbers will be used throughoutthis disclosure including the drawings to refer to the same or likeparts.

FIG. 2 is a planar view showing a flat panel display device with anoxide thin film transistor according to an embodiment of the presentdisclosure. FIG. 3 is a cross-sectional view showing the flat paneldisplay device taken along a line I-I′ in FIG. 2.

Referring to FIGS. 2 and 3, the flat panel display device with the oxidethin film transistor according to an embodiment of the presentdisclosure includes a thin film transistor array substrate 100 on whicha top gate thin film transistor is formed.

In detail, pixel regions P are defined by a plurality of gate lines 116and a plurality of data lines 118 which are arranged in a matrix shapeon a substrate 101. A thin film transistor is formed at an intersectionof the gate line 116 and the data line 118. The thin film transistor isused to switch a signal to be transferred to the respective pixel. Apixel electrode 121 connected to the thin film transistor is formed inthe pixel region.

The thin film transistor includes: a buffer film 102 formed on thesubstrate 101; a semiconductor layer 113, a gate insulation film 115 anda gate electrode 116 a sequentially stacked on the buffer film 102; andsource and drain electrodes 118 a and 118 b connected to thesemiconductor layer 113.

The semiconductor layer 113 has a width of a first length L1. Also, thesemiconductor layer 113 is formed from a compound containing oxygenions. A semiconductor thin film formed from the compound can be formedat a relative low temperature, and furthermore on a substrate such as aplastic plate, a plastic film or others. Also, the semiconductor thinfilm of the compound has an electron mobility ten times or more greaterthan that of amorphous silicon. As such, the semiconductor thin film ofthe compound is being spotlighted as a suitable element to realize ahigh definition of at least ultra-definition degree and a high speedoperation of above 240 Hz.

As an example of the compound forming the semiconductor layer 113, amaterial selected from a material group which includesamorphous-indium-gallium-zinc-oxide (a-IGZO),amorphous-indium-tin-zinc-oxide (a-ITZO), indium-gallium-oxide (IGO) andso on can be used.

The semiconductor layer 113 includes a channel region 113 a, and sourceand drain regions 114 a and 114 b. The source and drain regions 114 aand 114 b are formed through a process of metallizing the semiconductorlayer 113.

The gate insulation film 115 formed on the semiconductor layer 113 has awidth of a second length L2. The second length L2 is smaller than thefirst length L1 corresponding to the width of the semiconductor layer113, but larger than a third length L3 which is the width of the gateelectrode 116 a. As such, the present disclosure allows themetallization of the semiconductor layer 113 to be performed using onlythe gate insulation film 115 having the width of the second length L2without any additional mask procedure. Also, the channel region 113 a ofthe semiconductor layer 113 can be formed with sufficient length.

The gate insulation film 115 can be formed from a material selected froma material group which includes silicon oxide (SiO₂), silicon nitride(SiNx) and silicon oxynitride (SiON). The gate electrode 116 a can beformed from at least one material selected from a material group whichconsists of molybdenum (Mo), aluminum (Al), copper (Cu) and an alloy ofmolybdenum-titanium (MoTi).

An interlayer insulation film 117 is formed on the gate electrode 116 aover the entire surface of the substrate 101.

The source and drain electrodes 118 a and 118 b are formed on theinterlayer insulation film 117 in such a manner as to be connected tothe source and drain regions 114 a and 114 b of the semiconductor layer113, respectively. At the same time, a data line 118 connected to thesource electrode 118 a can be formed in the pixel region P, and a datapad 118 c connected to the data line 118 within the pixel region P canbe formed in a non-pixel region. The data pad 118 c is used to transfera signal from the exterior to the data line 118.

Subsequently, a passivation film 119 is formed over the entire surfaceof the substrate 101 including the pixel regions P. Also, a contact hole120 exposing a part of the drain electrode 118 b is formed in thepassivation film 119. Moreover, the pixel electrode 121 is formed on thepassivation film 119 in such a manner as to be connected to the drainelectrode 118 b through the contact hole 120.

FIGS. 4A through 4E are cross-sectional views illustrating step by stepa fabricating method of a flat panel display device with an oxide thinfilm transistor according to an embodiment of the present disclosure.

Referring to FIG. 4A, the method of fabricating the flat panel displaydevice with the oxide thin film transistor according to an embodiment ofthe present disclosure firstly allows not only a buffer film 102 to beformed on a substrate 101 but also a semiconductor layer 113 with thewidth of a first length L1 to be formed on the buffer film 112.

The semiconductor layer 113 can be formed from a compound semiconductorcontaining oxygen ions. In detail, the semiconductor layer 113 can beformed from an oxide material which is possible to form into a thin filmon a flexible substrate at a relative low temperature and has highmobility.

For example, one material selected from a material group which includesamorphous-indium-gallium-zinc-oxide (a-IGZO),amorphous-indium-tin-zinc-oxide (a-ITZO), indium-gallium-oxide (IGO) andso on can be used in the formation of the semiconductor layer.

The semiconductor layer 113 can be formed using one of a vapordeposition method and a photolithography method. For example, thesemiconductor layer 113 can be obtained by forming a thin film of IGZOand so on using the vapor deposition method and patterning the thin filminto a desired shape. The film formation process is employed for formingan oxide semiconductor sinter which is formed from one of IGZO and soon. As such, it is preferable to apply one of a sputtering method and apulse laser deposition method, which are included in the vapordeposition methods, to the formation of the semiconductor layer 113.More preferably, the sputtering method is applied to the formation ofthe semiconductor layer 113 for the mass production of the flat paneldisplay device.

The patterning process can be performed by forming a photoresist patternon the thin film of IGZO or others opposite to a formation region of thesemiconductor layer 113 and etching the thin film using an acid solutionsuch as hydrochloric acid, nitric acid, dilute sulfuric acid, a mixedsolution of phosphoric acid, nitric acid and acetic acid or others.Particularly, the solution containing phosphoric acid, nitric acid andacetic acid can remove an exposed portion of the IGZO film within ashort time.

As shown in FIG. 4B, a gate insulation material layer 115′ and a gateelectrode material layer 116′ are sequentially formed on the buffer film102 and semiconductor layer 113 over the entire surface of the substrate101.

Thereafter, a photoresist pattern 130 with the width of a second lengthL2 is formed on the gate electrode material layer 116′. The photoresistpattern 130 is positioned on a region of the gate electrode materiallayer 116′ opposite to a gate insulation film 115 which will be formedlater as shown in FIG. 4C. The width of the photoresist pattern 130having the second length L2 is almost equal to that of the gateinsulation film 115 which will be formed later as shown in FIG. 4C.

Referring to FIG. 4C, a gate electrode 116 a with the width of a thirdlength L3 is formed by etching the gate electrode material layer 116′using the photoresist pattern 130 as a mask. This etching process isperformed using a wet etching method.

If the distance between the gate electrode 116 a and the source anddrain electrodes 118 a and 118 b becomes shorter as shown in FIG. 4E, aparasitic capacitance is generated and furthermore the performance ofthe element (i.e., the thin film transistor) deteriorates. As such, thethird length L3 corresponding to the width of the gate electrode 116 ais set to an extent of preventing the formation of a parasiticcapacitance. However, the gate electrode 116 has a similar width to thatof an ordinary thin film transistor with almost equal performance.

On the other hand, the second length L2 corresponding to the width ofthe photoresist pattern 130 is larger than the width of a related artphotoresist pattern which is used to form the gate electrode of theordinary thin film transistor with almost equal performance. As such,the etching process using the photoresist pattern 130 as a mask can beperformed for a longer time, compared to another etch process using therelated art photoresist pattern.

Subsequently, a gate insulation film 115 is formed by etching the gateinsulation material layer 115′ using the photoresist pattern 130 withthe width of the second length L2 as a mask. At this time, the etchingprocess of the gate insulation material layer 115′ can be performedusing a dry etching method. Thereafter, the photoresist pattern 130 isremoved from the gate electrode 116 a.

The formed gate insulation film 115 has a width of the second length L2larger than that of the gate electrode 116 a. The source and drainelectrodes 118 a and 118 b do not cause any capacitance therebetween,unlike the gate electrode 116 a. As such, the performance of the element(i.e., the thin film transistor) is unchanged even though the width ofthe gate insulation film 115 becomes wider.

As shown in FIG. 4D, a metallization process is performed for thesemiconductor layer 113 using the gate insulation film 115 as a mask. Inaccordance therewith, source and drain regions 114 a and 114 b can beformed in the semiconductor layer 113.

As described above, the width of the gate insulation film 115 can be setwithout considering the parasitic capacitance. However, if the gateinsulation film 115 has the same width as the gate electrode 116 a, themetallized portion of the semiconductor layer 130 can expand under thegate electrode 116 a when the metallization process is performed. Due tothis, the width of a channel region 113 a can become narrower than thatof the gate electrode 116 a. This short-channel structure canconcentrate an electric field on the source and drain regions 114 a and114 b of the semiconductor layer 130 and cause a hat carrier effect. Inother words, the short-channel structure becomes a fault factor of theelement (i.e., the thin film transistor).

In view of this point, the present disclosure allows the gate insulationfilm 115 to be formed in a wider width compared to the gate electrode116 a.

Meanwhile, the gate insulation film 115 can have a wider width than thatof the semiconductor layer 113. In other words, the gate insulation film115 can be formed in such a manner as to cover the entire surface of thesemiconductor layer 113. Due to this, the gate insulation cannot be usedin the metallization process of the semiconductor layer 113 as a mask.

As such, the metallization process must be performed just after theformation of the semiconductor layer 113. To this end, a mask patternexposing portions of the semiconductor layer 113, in which source anddrain regions will be formed, is formed on the semiconductor layer 113.The mask pattern is removed from the semiconductor layer 113 after themetallization process is performed for the portions of the semiconductorlayer 113 which are exposed by the mask pattern.

The above-mentioned metallization process using an additional maskpattern requires an additional masking procedure, unlike themetallization process using the gate insulation film 115 as a mask. Dueto this, the metallization process using the mask pattern increasesfabrication time and costs.

In accordance therewith, the present disclosure sets the width of thegate insulation film 115 to be wider than that of the gate electrode 116a but narrower than that of the semiconductor layer 113.

The gate insulation film 115 includes tail portions not overlapping withthe gate electrode 116 a. As such, the metallized portions of thesemiconductor layer 113 cannot expand under the gate electrode 116 a.Therefore, the channel region 113 a with an effective width that is notless than the width of the gate electrode 116 a can be secured.

FIG. 5 is a data sheet illustrating enhanced properties of an element byapplying the fabricating method of the flat panel display device withthe oxide thin film transistor in accordance with an embodiment of thepresent disclosure. In other words, FIG. 5 illustrates, by comparison,properties of elements formed through a metallization process using thegate insulation film with the tail portions and another metallizationprocess using the gate insulation film without any tail portion.

Referring to FIG. 5, the fabricating method of the flat panel displaydevice with the oxide thin film transistor according to the presentdisclosure allows the elements (i.e., the oxide thin film transistors)to obtain a smaller distribution area and a narrower shift width ofthreshold voltages Vth in a small channel length range, compared tothose of the related art method. As such, the present disclosure canoptimize the range of the threshold voltage Vth of the elements.

For example, if the channel has a length of about 6 μm, the elements ofthe related art method have a mean threshold voltage of −2.2V and ashift width of the threshold voltage (i.e., a different of maximum andminimum threshold voltages) corresponding to 2.6V, but those formed bythe fabricating method of the present disclosure have a mean thresholdvoltage Vth of −0.9V and a shift width of the threshold voltage Vth(i.e., a difference of maximum and minimum threshold voltages Vth)corresponding to 0.4V.

Therefore, the fabricating method of the flat panel display device withthe oxide thin film transistor according to the present disclosure cansecure reliability and stability of elements (i.e., oxide thin filmtransistor) even though it is applied to a small-sized display devicewhich must shorten the length of channel.

Referring to FIG. 4E, an interlayer insulation film 117 is formed on theentire surface of the buffer film 102 and the gate electrode 116 a.Also, contact holes exposing the source and drain regions 114 a and 114b of the semiconductor layer 113 are formed in the interlayer insulationfilm 117 by patterning the interlayer insulation film 117.

Subsequently, source and drain electrodes 118 a and 118 b connected tothe semiconductor layer 113 are formed on the interlayer insulation film117. The source and drain electrodes 118 a and 118 b can be obtained bydepositing a metal film on the interlayer insulation film in such amanner as to fill the contact holes, and patterning the metal film. Themetal film can be formed from at least one of ruthenium Ru, zinc Zn,Indium In, tin Sn and so on,

Thereafter, a passivation film 119 is formed on the interlayerinsulation film 117 in such a manner as to cover the source and drainelectrodes 118 a and 118 b. Also, a contact hole 120 exposing a part ofthe drain electrode 118 b is formed in the passivation film 119.Moreover, a pixel electrode 121 connected to the drain electrode 118 bthrough the contact hole 120 is formed on the passivation film 119.

The flat panel display device with the oxide thin film transistoraccording to an embodiment of the present disclosure is a liquid crystaldisplay device. In this case, the flat panel display device furtherincludes a liquid crystal layer (not shown) and a color filter arraysubstrate (not shown) which are stacked on the thin film transistorarray substrate of FIG. 4E. Moreover, the flat panel display deviceincludes polarizing plates disposed on rear surfaces of the thin filmtransistor array substrate 100 and the color filter array substrate, anda backlight unit disposed under the thin film transistor array substrate100 as a light source.

Alternatively, the flat panel display device with the oxide thin filmtransistor according to an embodiment of the present disclosure can bean organic light emitting display device. In this case, the pixelelectrode 121 on the thin film transistor array substrate 100 shown inFIG. 4E becomes one of an anode electrode and a cathode electrode.

Also, an organic light emission layer is formed on the anode electrodeor the cathode electrode within each pixel region P and a sealingsubstrate configured to seal the pixel regions P is combined with thethin film transistor array substrate, thereby forming an organic lightemitting display device.

In this manner, the flat panel display device with the oxide thin filmtransistor of a top gate type can secure an effective channel length ofthe semiconductor layer. Therefore, the flat panel display device canenhance reliability of the element (i.e., the oxide thin filmtransistor).

Also, the method of fabricating the flat panel display device with theoxide thin film transistor according to an embodiment of the presentdisclosure can simplify the fabricating procedure and reduce the numberof mask procedures. As such, not only productivity can be enhanced butalso fabricating costs can be reduced.

Although the present disclosure has been limitedly explained regardingonly the embodiments described above, it should be understood by theordinary skilled person in the art that the present disclosure is notlimited to these embodiments, but rather that various changes ormodifications thereof are possible without departing from the spirit ofthe present disclosure. Accordingly, the scope of the present disclosureshall be determined only by the appended claims and their equivalentswithout being limited to the detailed description.

What is claimed is:
 1. A flat panel display device with an oxide thinfilm transistor, the device comprising: a buffer film on a substrate; anoxide semiconductor layer of the oxide thin film transistor which has awidth of a first length and is disposed on the buffer film, the oxidesemiconductor layer having a source region and a drain region; a gateinsulation film of the oxide thin film transistor which has a width of asecond length and is disposed on the oxide semiconductor layer, the gateinsulation film having a first end over the source region and a secondend over the drain region, and the gate insulation film having a topsurface parallel with and facing away from the substrate; a gateelectrode of the oxide thin film transistor which has a width of a thirdlength and is disposed on the gate insulation film, wherein a portion ofthe top surface of the gate insulation film extends away from the gateelectrode; an interlayer insulation film on the gate electrode over anentire surface of the substrate; source and drain electrodes disposed onthe interlayer insulation film and connected to the oxide semiconductorlayer, the source electrode separated from the first end of the gateinsulation film by the interlayer insulation film, and the drainelectrode separated from the second end of the gate insulation film bythe interlayer insulation film; a passivation film on the source anddrain electrodes over the entire surface of the substrate; and a pixelelectrode disposed on the passivation film and connected to the drainelectrode, wherein the first length is larger than the second length andthe second length is larger than the third length.
 2. The flat paneldisplay device of claim 1, wherein the oxide semiconductor layerincludes a channel region, and wherein the gate electrode overlaps withthe channel region.
 3. The flat panel display device of claim 1, whereinthe gate electrode comprises at least one of molybdenum (Mo), aluminum(Al), copper (Cu) and an alloy of molybdenum-titanium (MoTi).
 4. Theflat panel display device of claim 1, wherein the gate insulation filmcomprises a material selected from a material group consisting ofsilicon oxide (SiO₂), silicon nitride (SiNx) and silicon oxynitride(SiON).
 5. The flat panel display device of claim 1, wherein the oxidesemiconductor layer comprises a material selected from a material groupconsisting of amorphous-indium-gallium-zinc-oxide (a-IGZO),amorphous-indium-tin-zinc-oxide (a-ITZO) and indium-gallium-oxide (IGO).